This invention is directed to a solid, thermoplastic composition which is conductive to ions while being an electrical insulator. More particularly, the composition comprises a plasticized terpolymer of vinyl butyryl, vinyl alcohol, and vinyl acetate and alkali metal salt.
Liquid electrolytes have conventionally been used in electrochromic devices, batteries, sensors, and capacitors. Electrochromic devices comprise an assembly of parallel layers of materials including electrodes and therebetween an electrochromic material and an electrolyte. Upon providing a current across the electrodes, the electrochromic material changes color. Such devices have been suggested to moderate light transmittance in sunroofs or windshields. When liquid electrolytes are employed in electrochromic devices, there is the possibility of leakage of the electrolyte out of the device, the inherent difficulty of sealing the device, and the obvious lack of mechanical strength and adhesion of the electrolyte layer to the adjacent layer.
Various solid electrolytes have been suggested for use in electrochromic devices. Solid electrolytes of complex halides are known, particularly iodides of silver with an alkali metal. Additionally, solid electrolytes may comprise aluminum compounds such as sodium beta-alumina and potassium beta-alumina. However, these electrolytes are all typically expensive to prepare and, in the case of the alumina compounds, could not be formed directly on components of an electrochromic device since they require very high processing temperatures. Because of the rigidity of solid materials, it is also difficult to assemble the component layers of an electrochromic device and achieve the intimate contact required between the solid electrolyte and the adjacent layers, particularly if the surfaces of the device are curved.
Such problems are minimized with polymeric electrolytes which are generally either extremely viscous liquids or tacky solids. It would be desirable, however, if the polymeric electrolytes were solids which could be formed into self-supporting films with a degree of flexibility to allow uncomplicated device fabrication.
One approach to forming such a flexible solid material involves copolymerization of polyethylene oxide and polypropylene oxide and results in systems having good conductivity at room temperature, but a markedly reduced ionic conductivity at low temperatures. Another approach to the formation of room temperature flexible solid polymer electrolytes includes forming materials having backbones of phosphazene and siloxane-based polymers with etheric side chains. These materials generally lack appreciable dimensional stability and must be chemically or radiationally cross-linked. In addition, the susceptibility of the Si--O--C bonds to hydrolysis and subsequent structural degradation constitute a severe problem unless moisture can be entirely eliminated.
Yet another approach is directed to a complex of poly(2-methoxy polyethylene glycol monomethacrylates) with a lithium salt of triflic acid having etheric side chains of 9-22 ethylene oxide units. Similar polymers copolymerized with styrene have also been suggested. Common problems encountered by each of these is the inverse relationship between optimization of the ionic conductivity and simultaneously the mechanical properties. The dependence of ionic conductivity on salt concentration in amorphous materials must also be optimized. Decreasing the salt content, while it desirably reduces crystallinity may also reduce the mechanical strength and ionic conductivity of the resulting material. In these and similar systems, purification and neutralization steps necessary in processing such materials is very time-consuming.
The present invention provides a thermoplastic ionic conductor that overcomes the deficiencies of the foregoing materials in that it is easily prepared, exhibits high temperature stability, requires no curing reaction, forms tough flexible free-standing films easily applied to a surface with curvature, shows strong adherence to glass and coatings, and may be used in current manufacturing processes for laminated glass products. It is a thermoplastic material able to perform well in an electrochromic-laminated glass systems, including automotive windshield applications.